For analyzing microwave circuits and devices of vastly different kinds, from filters, amplifiers etc. to much more complex multifunction systems, Vector Network Analyzers, VNA:s, are widely used. Such an instrument is capable of evaluating the characteristics of a circuit or a device to be analyzed through measuring the scattering parameters (S-parameters), and operate as a signal source and a signal receiver measuring magnitude and phase and magnitude of transmission and reflection, i.e. the S-parameters.
However, it is of utmost importance to be able to calibrate the VNA in order to provide useful measurement results of the highest accuracy. Generally a calibration is performed using a number of known standards, which are measured and transferred to the VNA, and errors hence removed from the VNA.
Therefore it is known to use so-called calibration kits comprising a number of standards. A number of standards are connected to the ports of a VNA to perform and verify calibrations. This is a very time-consuming process requiring expert skill and high precision. The calibration process is therefore prone to errors.
Moreover, the use of high frequencies is steadily gaining more interest. Up to about 67 GHz coaxial cables can be used as transmission lines, and therefore standard coaxial connectors are used on the calibration kits. However, for frequencies above 67 GHz rectangular waveguides are used instead of cables, and therefore the calibration kits are provided with standard waveguide flanges to be used as connectors to the equivalent flanges on the circuit or device to be tested.
For lower frequencies, when coaxial cables are used, known calibration kits comprise a plurality of coaxial connector standards. The coaxial cable technology makes it possible to use flexible cables with connectors on both ends between the calibration kit and the VNA. Such flexible cables make the measurement setup easy to handle.
However, for higher frequencies, e.g. from about 67 GHz up to about 1 THz, when waveguides need to be used, the situation is much more complicated. The rectangular waveguides are normally rigid, and when connecting two opposing waveguide flanges together there is required a high quality of both mechanical and electrical contact between them, in order to obtain a high quality, a repeatable and non-radiating interface, and thereby also a good calibration.
Known calibration kits consist of waveguide calibration standards such as matched loads, short circuited lines of different lengths (referred to as shorts), and straight waveguides of given lengths (referred to as throughs). These are screwed on to or between the ports of the network analyzer (VNA) in a calibration sequence. In a typical calibration procedure the operator has to connect the flanges of the calibration standards and the ports of the VNA together a plurality of times. This is a very time consuming, complicated and tedious task, due to all the screws needed to make good contact between all joining flanges. It requires a stable and repeatable contact both mechanically and electrically, and therefore four screws are always used. If the connection is not perfect, e.g. if there is a slight angular displacement or if there is not a perfect fit, there may be a leakage from the waveguide into free space, and also increased reflection at the connection. The calibration procedure is based on all such connections being as perfect as possible, for the most accurate measurements.